In a scanning electron microscope, the specimen to be examined is scanned by an electron beam focused onto the specimen. During this operation, various interactions take place in the specimen. The most important of these are low-energy secondary electrons (SE) and electrons of the electron beam scattered at the specimen with little or not energy loss, which are known as back-scattered electrons (BSE). With the aid of suitable detectors and an image formation corresponding to the scanning movement of the electron beam, images are produced, for instance on a monitor, which correspond to the electron emission of the specimen. Since in the case of flat specimens, the back-scattered electron coefficient is a function of the atomic number Z, it is possible, using detectors that respond only to back-scattered electrons, to perform material analyses in micro ranges (so-called material contrast).
European patent application EP-Al No. 0 018 031 discloses a detector system which includes a plurality of detector elements having flexible optical light conductors. At one end of each light conductor, there is an optical funnel with an entrance screen. The other ends of the light conductors are coupled to a transparent block that serves as a wall sealing means. The detector system includes selection means for selecting from signals from at least one of the detector elements. In order to measure the back-scattered electrons, the detector elements must be located between the last electron optical lens and the specimen. With these detector elements, however, this spacing cannot be as close as is required for short focal-length objectives that are needed for high resolution.
From a publication by Autrata et al in Scanning Electron Microscopy 1983/II, page 489, the use of cerium-doped YAP and YAG monocrystals is known. In this article, detectors having wafer-like monocrystals are described, which have a bore in the center for the electron beam and can therefore be accommodated in a narrow space between the last electron optical lens and the specimen. The monocrystals are connected via light-conducting devices to photomultipliers. If the monocrystal is circular in shape, one-half of the circular side wall is cemented to the light-conducting device. If the monocrystal is square, one side face is cemented to the light-conducting device, and an AL.sub.2 O.sub.3 layer having a thickness of .lambda./4 effects a better transfer of photons into the waveguide device. In another embodiment having a circular monocrystal, one circular face rests on the flat part of a unidirectional light-conducting device, which makes it possible to attain a stronger output signal for the detector.
These known detectors having monocrystals have the disadvantage that with homogeneous specimens, the electrons that are scattered back from different points trigger signals of various intensities; especially images at low magnifications, this results in evenly illuminated images. This disadvantage becomes particularly problematic when material analyses are performed.